Method for extruding molybdenum and tungsten



1967 R. E. MCDONALD ETAL 3, 07

METHOD FOR EXTRUDING MOLYBDENUM AND TUNGSTEN Filed Nov. 6, 1964 FORCEINVENTORS. Robert E. M Donald Carl F. Le/Hen, Jr.

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ATTORNEY United States Patent Office 3,350,907 METHOD FOR EXTRUDINGMOLYBDENUM AND TUNGSTEN Robert E. McDonald and Carl F. Leitten, Jr., OakRidge, Tenn., assignors to the United States of America as representedby the United States Atomic Energy Commission Filed Nov. 6, 1964, Ser.No. 409,620

6 Claims. (Cl. 72-38) extruded, but not without some difiiculty. Due totheir lack of plasticity at low temperatures, these metals generallyrequire that the extrusion be carried out at an elevated temperature, aswell as requiring some type of lubricant. Various types of lubricants,such as graphite, graphite in oil, glass, and highly ductile metals,have been tried, but have not been to successful with these high meltingmetals. To illustrate, molybdenum and tungsten metals require such highextrusion temperatures (2800 3200 F.) that most glass and metal compoundlubricants have been found to be ineffective, probably due to the lackof adherence at the extrusion temperature. Also, highly ductile metalsheathing materials have been found undesirable in many suchapplications due to poor boundary. layer lubrication properties atelevated temperatures, expense of sheathing materials and canningoperation, and alsodue to the likelihood of a reaction between thesheathing and the extrusion billet. It may thus be seen that it would behighly desirable to find a lubricant which is compatible with carryingout high temperature extrusions of metals like molybdenum and tungsten.

It is therefore a general object of the present invention to provide amethod for extruding metals which form volatile oxides at the extrusiontemperature without the use of additive lubricants.

Another object is to provide an improved method for extruding molybdenumand tungsten metals and alloys thereof at elevated temperatures whereinthe resulting product is essentially free of all contaminants.

Still another object is to provide an improved method for fabricatingmolybdenum and tungsten metal tube shells which have a finish superiorto that normally obtained by prior art methods.

Other objects and advantages of this invention will become apparent fromthe following detailed description thereof when read in conjunction withthe accompanying drawing, in which:

FIG. 1 is a schematic View of one embodiment of the assembled extrusionapparatus.

In accordance with the present invention there is provided an improvedmethod for extruding metals which form volatile oxide bearing surfacesat the extrusion temperature comprising the steps of heating the metalto the extrusion temperature, exposing the heated metal to an oxidizingatmosphere immediately preceding the extrusion operation to therebycause the metal to undergo a surface oxidation and thereafter hotextruding the metal. Applicants have found, quite unexpectedly, that therefractory metals molybdenum and tungsten, as well as alloys thereof,can be extruded at elevated temperatures Patented Nov. 7, 1967 withoutany additive lubricant, using instead the volatile oxide, which isformed by exposure to such oxidizing environments as air, as alubricant. Whereas with usual lubricants there is a contaminationproblem after the extrusion, the present process is essentially free ofany such problem inasmuch as the oxide formed with molybdenum andtungsten is volatile at the extrusion temperature and is removed almostas fast as it is formed. Furthermore, the resulting tube shells areamenable to subsequent drawing into metal tubing.

Preparation of the metal billet may be accomplished by any suitablemanner such as by powder metallurgy techniques or arc-casting. Forexample, tungsten metal billets may be prepared by pressing tungstenmetal powder into compacts and sintering the compacts. After cooling,the sintered product may be ground to the desired dimensions of thebillet. On the other hand, molybdenum may be prepared in wrought form byarc-casting the metal into a suitable form and then hot-rolling themetal to the desired dimensions. While the finished dimensions of thebillet are not critical and may be varied over a wide range to meet therequirements of the extrusion press, applicants have found certaindimensions to be preferred. Cylindrical metal billets (3"-6" OD. x 512"long) having a one-inch axial bore therethrough have been found to bequite suitable for extruding metal tube shells and thus are preferred.

The extrusion of the metal billets into tube shells may be carried outwith any conventional extrusion apparatus. Referring to FIG. 1, theapparatus may comprise a cylindrical billet container 1 adapted toreceive the refractory metal billet 2 which has an axial boretherethrough. A thin-walled metal liner 3 is removably mounted withinthe billet container 1. A die assembly 4 consisting of a conical die 5,die holder 6, and die bolster block 7 is mounted to one end of thebillet container 1. A floating mandrel 8 which has its terminal endthreaded is adapted to be centered in the billet container 1 so as toextend therethrough the axial bore of the refractory metal billet 2 andaxially aligned with the die 4. This centering of the floating mandrel 8may be accomplished by screwing the threaded end of the mandrel into adummy block 9 which is slidably retained within the billet container 1.A follower block 10 having an axial bore therethrough may be slid overthe mandrel in abutting relation with the dummy block 9 and the tail endof the billet 1. An extrusion ram 11 is adapted to be slidably receivedin the billet container 1 in abutting relation with the dummy block 9.Suitable heater means 12 may be placed around the billet container 1 toheat the liner, container, and die assembly to a maximum temperature ofabout 1000 P. which is below the temperature at which the steelcomponents would soften. The extrusion ram 11, die 5, and mandrel 8should be made of hardened alloy steel, such as H42, and the die,mandrel, and follower block should be coated with suitable insulatormaterial, such as zirconia, to insulate these parts from the heatedmetal billet, thus precluding melting and/or reacting of these partsduring the extrusion.

In carrying out the practice of this invention the conditioned metalbillet is heated to the extrusion temperature in a suitable furnace. Forthis an argon atmosphere induction furnace has been found to be quitesuitable. It will be appreciated that the extrusion temperatures for therefractory metals, molybdenum and tungsten as well as their alloys, mayvary over a Wide range. As is known in the extrusion art it is essentialthat the extrusion be carried out within a range of temperatures inwhich the metal has sufficient plasticity to allow the'shaping processto be conducted within the capacity of the available press. Also, itshould be carried out within a range of temperatures in which theeffects of the deformation are dissipated rapidly enough to prevent theresistance of the material from being affected to any serious extent bywork-hardening such as occurs at lower temperatures. Such a temperaturerange will depend, for example, not only upon the particular metal, butalso upon the reduction ratio employed and the capacity of the extrusionpress.

After the metal billet is heated to the extrusion temperature it isremoved from the furnace and inserted immediately in the billet chamberof the extrusion press. Upon contact with room air the molybdenum metal,for example, was found to undergo rapid oxidation, evolving white cloudsof molybdenum oxide from the billet surface. This was also observed inthe case of tungsten metal when exposed to ambient air after beingheated to the extrusion temperature. Upon completion of the transfer ofthe billet to the extrusion press, the billet is extruded into metaltube shells which could then be fabricated into metal tubing byconventional metal fabrication techniques, as for example, by warmdrawing. Inasmuch as the oxidation of the metal tubes took place almostinstantaneous upon contact with an oxidizing atmosphere, the length ofexposure of the heated metal to oxidizing environments is not critical.Applicants have found that the time lapse between removal of the billetfrom the furnace and completion of the extrusion was about seconds. Itwill be appreciated that the time lapse should not be too great or themetal billet might have to be reheated to insure maintaining a desiredtemperature during the extrusion operation.

The extrusion press parameters are not critical in the practice of theinvention. For example, reduction ratios employed varied from 6 to 1 fortungsten metal billets to 9 to 1 for molybdenum metal billets and theram speeds were in both cases 300 inches/ minute. The press capacity was700 tons and the press had a stroke of 18 inches.

It is not completely understood how the volatile oxide affords goodlubrication of the metal billet during extrusion. However, it isbelieved that the metals, to witmolybdenum and tungsten-undergo surfaceoxidation at a rate sufficient to provide a suitably lubricated bearingsurface for extrusion. To insure a suitably lubricated bearing surface,it is believed important that the oxide be liquid during the extrusion.While the extrusion should be carried out at a temperature above themelting point of the particular oxide, the extrusion temperature shouldbe selected to insure that the oxide present is at an optimum amountthroughout the extrusion. Applicants have found that too little oxidewill provide no lubricating bearing surface and too much oxide mayresult in surface defects and inferior surface finishes.

Inasmuch as the oxide is volatile it will be appreciated that theevaporation rate of the oxide has a significant effect upon theeffectiveness of the oxide bearing surface as a lubricant duringextrusion. Applicants have found, for example, that when a tungstenbillet was extruded at a temperature of about 3400 F. the breakthroughpressure was 480 tons, whereas when a tungsten billet was extruded at3200 F. the breakthrough pressure was significantly lower (-405 tons).It further was noted that the rate of evaporation of W0 at 3400 F. wasabout 7 /2 times as great as that at 3200 F. Thus, it is believed thatat the high extrusion temperature (3400 F.) the rate of evaporation ofthe oxide was too great to afford an adequate bearing surface. However,even at 3400 F. the surfaces of the resulting billets were found to befar superior to those produced with such lubricants as glass.

Further illustration of the quantitative aspects and procedures of thepresent invention is provided in the following examples. Examples I-IIIdemonstrate the applicability of this invention to fabricatingmolybdenum and molybdenum alloy tube shells and/ or metal tubing fromtube hollows.

4 Example I A wrought unalloyed molybdenum metal billet which had beenarc-cast 'was machined to 3" OD. x 5" long billet and had a axial boretherethrough. One end of the billet was machined to a cone shape havinga included angle. All finished surfaces were polished to a finish ofbetter than 64 R.M.S.

The extrusion press utilized for extruding the molybdenum metal billetinto a tube shell had a press capacity of 700 tons and consisted ofhardened H-12 tool steel, zirconia-coated floating mandrel and 90conical die. The press had a ram speed of 300 inches/minute and an 18"length stroke. Heating coils were placed around the billet container toheat the liner, container and die assembly to a temperature of 1000 F.Prior to the insertion of the billet in the billet container, the dieand mandrel were sprayed with colloidal graphite and the excess wipedoff.

The molybdenum metal billet was placed in an induction furnace andheated in an argon atmosphere to an extrusion temperature of 3000 F.When the billet was at temperature, it was removed from the furnace andinserted immediately into the billet container. Upon contact with theroom air, white clouds of molybdenum oxide were evolved from the billet.The billet was then extruded at a reduction ratio of 8.8 to 1 into amolybdenum tube shell (1%" OD. x 40 long) with the time lapse betweenremoval of the billet from the furnace and completion ofthe extrusionbeing about nine seconds.

Visual inspection of the tube shell indicated a honed appearance withgood finishes, and essentially no trace of contaminants. Subsequentultrasonic inspection of the billet indicated no significant defects inthe tube shell.

After the tube shell was inspected for defects it was placed on awarm-draw bench and drawn to O.D. molybdenum tubing having a wallthicknes of 0.035 inch by conventional warm-drawing techniques.

Example 11 A wrought molybdenum alloy-TM (Mo-0.5% Ti) metal tube hollowbillet was prepared and extruded into a tube shell (1%" OD. x 40" long)as in Example I. The extrusion was carried out at a temperature of 3500F., ram speed of 300 inches/minute, and a reduction ratio of 8.8 to l.The resulting tube shell was visually examined for contaminants,ultrasonically checked for defects, and was found to be free of both.The tube shell was then warm drawn into O.D. metal tubing having a wallthickness of 0.035 inch. This is the first known defect-free molybdenumalloy-TM tubing to be drawn from an extruded tube shell.

Example III A wrought molybdenum alloy-TZM (Mo-0.5% Ti- 0.1% Zr0.08% C)metal tube hollow billet was prepared and extruded into a tube shell(1%" OD. x 40 long) as in Example I. The extrusion was carried out at atemperature of 3700 F., ram speed of 300 inches/ minute, and a reductionratio of 8.8 to 1. The resulting tube shell was visually examined forcontaminants, ultrasonically checked for defects, and was found to befree of both. The tube shell was then warm drawn into O.D. metal tubinghaving a wall thickness of 0.035 inch.

Examples IV and V demonstrate the applicability of the present processfor extruding tungsten tube shells from tube hollows.

Example IV A sintered tungsten billet, 3" CD. by 5" long having an axialbore 1 /8" in diameter and a conical nose with a 90 included angle, wasprepared for extrusion as in Example I. The billet was heated to atemperature of 3400 F. in an argon atmosphere, When the billet reachedtemperature, it was transferred immediately to the billet container.Upon coming in contact with ambient air, a white cloud of W0 formedaround the billet. The billet was extruded at 3400 F. with a 5.88 to 1reduction ratio to form a tube shell approximately 18" long and 1 /8"OD. The billet had a breakthrough pressure of 480 tons and a runningload of 405 tons and a ram speed of 300 inches/minute.

After extrusion the tube shell was visually examined and the outsidesurface was found to have areas of roughened appearance with a surfacefinish which was only of average appearance.

The resulting tungsten tube shell was then Warm drawn into O.D. tungstentubing having a Wall thickness of 0.035 inch.

Example V A sintered tungsten billet, 3 OD. x 5" long having an axialbore 1 /8" in diameter and a conical nose with a 90 included angle, wasextruded as in Example I, except it was extruded at 3200 F. The billethad a breakthrough pressure of 405 tons and a running load of 380 tons.The resulting tube shell (18" x 1 /3" CD.) was visually examined and allof the extruded surfaces were found to be of superior appearance.Subsequent nondestructive evaluation indicated no significant defects inthe tube shell. The tube shell was then warm drawn into /1" OD. tungstentubing having a Wall thickness of 0.035 inch.

Examples VI and VII demonstrate that solid tungsten billets may beextruded with the present invention to produce wrought tungsten barstock.

Example VI A sintered powder metallurgy solid tungsten billet (6'' OD. x11 long) having a conical nose wtih a 90 included angle was extrudedbare as in Example I, except it was extruded at 2850 F. with a reductionratio of 4: 1. The extruded bar stock (3" OD. x 44" long) was visuallyexamined and found to have an excellent surface with no nose burst, andultrasonically checked and found to be essentially free of any defects.This wrought bar stock may be subsequently conditioned to tube hollowbillets for ultimate tube shell production.

Example VII A sintered powder metallurgy solid tungsten billet (6'' OD.x 11" long) having a conical nose with a 90 included angle Was extrudedbare as in Example I, except it was extruded at 3200 F. with a reductionratio of 4:1. The extruded bar stock (3" OD. x 44" long) was visuallyexamined and found to have a good surface with minimum nose burst. Thiswrought bar stock may be subsequently conditioned to tube hollow billetsfor ultimate tube shell production.

From the results shown, it appears that the extrusion of tungsten metalbillets utilizing the surface oxidation of the tungsten metal to providean oxide bearing surface is better carried out at the lower temperature.The breakthrough pressure at the lower temperature was found to be lessthan that at the higher temperature which is the reverse of what mightbe expected.

It is to be understood that the foregoing examples are merelyillustrative and are not intended to limit the scope of this invention,but the invention should be limited only by the scope of the appendedclaims.

What is claimed is:

1. An improved method for extruding a metal selected from the classconsisting of molybdenum, tungsten, and alloys thereof which comprisesthe steps of heating said metal to an extrusion temperature within therange of 2850-3700 F., exposing said heated metal to an oxidizingatmosphere immediately preceding the extrusion operation thereby forminga volatile oxide bearing surface and thereafter extruding said metal.

2. The method of claim 1 wherein said metal is molybdenum, saidextrusion is carried out at a temperature of 3000 F., and said oxidizingatmosphere is ambient air.

3. The method of claim 2 wherein said molybdenum metal is an arc-casttube hollow billet.

4. The method of claim 1 wherein said metal is tungsten, said extrusionis carried out at a temperature of 2850 F., and said oxidizingatmosphere is ambient air.

5. The method of claim 4 wherein said tungsten metal is a sintered tubehollow billet.

6. The method of claim 4 wherein said tungsten metal is a sintered solidtungsten billet.

1. AN IMPROVED METHOD OF R EXTRUDING A METAL SELECTED FROM THE CLASSCONSISTING OF MOLYBDENUM, TUNGSTEN, AND ALLOYS THEREOF WHICH COMPRISESTHE STEPS OF HEATING SAID METAL TO AN EXTRUSION TEMPERATURE WITHIN THERANGE OF 2850*-3700*F., EXPOSING SAID HEATED METAL TO AN OXIDIZ-